This invention is in the field of detection of electrically-conducting objects buried at shallow depths in the ground, in particularly under the sea, by active electromagnetic induction.
The invention can be applied in particular to the detection of underwater mines laid near coasts in relatively shallow water. Most of the time, due to their weight and the action of waves and tides, these mines are buried in coastal sediment and sands.
Another example is for archaeological research for metallic debris buried over the course of time in the sands and sediments of sea and river beds.
Classic NMR magnetometry methods can be used to locate shallow-buried magnetic objects in some cases. Their limitations are due to surrounding noise, false alarms due to any other magnetic objects as well as the strong signal decrease with distance.
Acoustic detection techniques (sonar) developed for the same type of ground applications are not very effective in the sea [1] because, if high frequencies are used, the attenuation of the waves is very great in the sea and in sediments saturated with water. If low frequencies are used, the instrument""s directivity is low and there are many interfering echoes from the roughness of the sea bed and the sediment geology which mask the xe2x80x9cusefulxe2x80x9d signal.
Classic radar detection techniques [2] GPR (ground penetrating radar) are not effective because of the high absorption of high frequency electromagnetic waves by the water layer and by sediments saturated with water.
It is often impossible to detect these objects by optical methods because they are buried orxe2x80x94even if the objects are not totally buriedxe2x80x94because of the turbidity of the sea bed, particularly in coastal areas, due to waves and swell.
Active electromagnetic methods have recently been developed for detection of these types of objects. The following paragraph explains the principle of these methods, examples of existing patents, and their limitations.
The detection principle is based on electromagnetic induction. More precisely, pulses of current circulating in a transmitting antenna induce Foucault currents which are concentrated in the most conducting parts of the groundxe2x80x94particularly in the objects to be detected. Once the current is cut off in the transmitter antenna, the induced currents then provoke an electric current in the receiving antenna by induction. The principle is thus to electrically charge the inhomogeneous conducting elements in the soil which then transmit some of this energy to the receiving antenna. The response signal to the receiving antenna is then analysed to diagnose the presence or absence of a metallic object or electrical conductor. FIG. 1 provides a diagram of the electric current when the transmitting antenna also acts as the receiving antenna.
In this figure, the current pulses emitted by the antenna are shown by full lines, The shape of the return signal received is shown by the dotted line having the shape of the dotted line b when there are no buried metal objects and dotted line c when an object is buried. The return signal received is of a higher level in this case.
An example of the use of such a process is described in the patent U.S. Pat. No. 5,654,637 awarded to GEONICS [3] entitled xe2x80x9cMethod for detecting buried high conductivity objects including scaling of voltages for eliminating noise of a particular depthxe2x80x9d. This patent describes an electromagnetic pulse system to detect metallic objects in the ground. The operating principle is that explained above: electrical current pulses induce currents in the metallic object to be detected. The reception antenna system is made of two horizontal antennae located along the same vertical line. This system reduces noise and gives an estimation of the depth of the object.
This patent is intended for applications for searching for metallic objects and parts in the ground. It is not directly applicable to xe2x80x9chostilexe2x80x9d media such as the sea, because the response signal recorded by the second loop (second receiving antenna) is too weak due to absorption of the field by the sea water.
Another example of the use of such a process is described in the patent application WO 98/08106 of February 1998 by KELLER [4] entitled xe2x80x9cnon obtrusive weapon detection system and method for discrimination between a handgun and other metal objectxe2x80x9d. This is used for detecting weapons carried by people at airports.
The principle of the system is the same: a current-carrying transmitter induces electric current in the object to be detected. Rapid magnetic sensors then register the time response. The time constant characteristic of the decrease in the response allows for differentiation of various metallic objects which people may be carrying in public places. The measured value of the time constant is compared with constant values listed in a database.
The process described in this patent rightly assumes that the position of the object with respect to the antenna loop is known. It is only concerned with the time constant of the decrease.
Such a process is not applicable to objects of unknown shapes located at unknown distances.
There is also the patent application [5] of P. ELLIOT No. WO 92/19989 of November 1992 entitled: xe2x80x9cAirborne transient electromagnetic method with ground loopsxe2x80x9d. This patent application is mainly intended for applications such as petroleum prospecting because it describes a device with a very large transmitter and an airborne receiver.
This patent application [5] focuses particularly on geology and geophysics: the dimensions are too large to detect mine-sized objects. Furthermore, the system cannot work in a marine environment.
The state of the art which has just been described shows that when the media where the transmission and reception antennae are placed are poor conductors, the systems and processes of the prior art do not work. For the sea in particular, as sea water is conducting, much of the energy supplied is dissipated in the marine environment and only a small part constitutes the xe2x80x9cusablexe2x80x9d signal. A much more sensitive system must therefore be used, allowing for processing of the signal for detection analysis.
According to the invention, in an initial phase a database including the complete temporal response or only certain values characteristic of this response for any three-dimensional conducting objects with a shape similar to that of the objects sought is generated. For example if it is a sea mine, the temporal response of an object with exterior conducting parts corresponding to that of the mines sought is calculated or measured during a test campaign. Then, during a search phase:
a series of electromagnetic pulses is transmitted by an antenna in the form of at least one loop
for each pulse, the temporal response received in return by a receiving antenna is recorded,
an average temporal response is created by taking the average of the various responses received. Some of the distortions of the individual curves due to electromagnetic noise of the sea which is relatively substantial due to the weakness of the signal are thus eliminated.
The processing of the average decreasing curve to identify the presence of a object sought, its nature and its location can then be approached in several ways. These involve comparing the characteristics of the average decreasing curve with the same characteristics of various decreasing curves obtained in a previous phase. The inventors determined that the probability of a false alarm by comparison solely with the time constant, as in the method described in the patent request already cited [4], was too high in marine media.
In a first form of the processing, the selected characteristics of the average decreasing curve are composed of the level values of the signal received, at the moments of sampling of this signal, these characteristics thus forming an average real decreasing curve. According to this first type of processing:
a value representing the difference between the average real decreasing curve and each of the decreasing curves of the pre-recorded digital signals is calculated,
the smallest of the values representing the difference between the average real decreasing curve and each of the pre-recorded decreasing curves is compared with a threshold value,
there is detection if at least one value representing a difference between this average decreasing curve and one of the pre-recorded decreasing curves is less than a pre-determined threshold, the object and its buried depth being defined as the object from the database which gave the value of the minimum difference.
In a second type of processing, the selected characteristics of the average decreasing curve are composed of measured or calculated characteristics which define certain aspects of this curve. These characteristics are compared with the same characteristics of various decreasing curves obtained in a prior phase. This second type of processing has, with respect to the first, the advantage of reducing the size of the database, because only the characteristics of each curve within the selected set are stored. This also allows for more rapid processing.
In its first embodiment, the invention thus involves a process for detection and localisation of electrically-conducting objects buried in the ground, the process involving the sending of a series of electromagnetic pulses into the medium to be explored by means of a transmitting antenna with at least one loop, and comparing a return signal received on a receiving antenna having at least one loop, with at least one signal characteristic stored in a process database characterised in that:
the characteristics of the signals stored in the database are composed of numeric data which each define, in the temporal domain, the shape of a decreasing curve of the value of the signals received in a phase prior to the search, by a first receiving antenna for formation of the database, above ground in which an object is buried and which has an exterior conducting shape identical or similar to an object sought, for various sizes and object depths, after transmission of signals by a first transmitting antenna for formation of the database, and then in the search phase,
the numerical values are periodically recorded for the samples of rank 1, 2 . . . n, together representing a real decreasing curve of a signal received by a detection receiving antenna having the same shape and orientation as the receiving antenna used to create the database, this signal being received after transmission by a real antenna having the same shape and orientation as the transmitting antenna which was used to form the pre-recorded characteristics,
an average decreasing curve of an average signal received is created by determination of average sample values, each of which represents the average value of the values of samples of the same rank,
a value representing the difference between the real average decreasing curve and each of the decreasing curves of the pre-recorded digital signals is calculated,
the smallest of the values representing a difference between the real average decreasing curves is compared with each of the pre-recorded decreasing curves,
there is detection if at least one value representing a difference between this average decreasing curve and one of the pre-recorded decreasing curves is below a given threshold, the object and its buried depth being defined as the object of the database which had the minimum difference value.
In its second embodiment, the invention involves a process for detection and localisation of electrically-conducting objects buried in the ground, the process involving the sending of a series of electromagnetic pulses into the medium to be explored by means of a transmitting antenna with at least one loop, and comparing a return signal received on a receiving antenna having at least one loop, and at least one signal characteristic stored in a process database characterised in that:
the characteristics of the signals stored in the database are composed of points in a space of N dimensions, N being a whole number greater than or equal to 1, the dimensions representing the characteristic values of a decreasing curve over time of the value of the signals received in a phase prior to the search, by a first receiving antenna for formation of the database, above ground in which an object is buried which has an exterior conducting shape which is identical or similar to the object sought, for various sizes and depths of the object, after transmission of pulses by a first transmitting antenna for formation of the database, and
a detection volume is demarcated in the space in N dimensions, this volume being demarcated by a surface enveloping all points contained in the database, then, during a search phase,
numerical values of samples of rank 1, 2 . . . n are periodically recorded which together represent a real decreasing curve of a signal received by a detection receiving antenna having the same shape and orientation with respect to the ground as the receiving antenna which was used to create the database, this signal being received after transmission by a real antenna having the same shape and the same orientation with respect to the ground as the transmitting antenna which was used for formation of the pre-recorded characteristics,
an average decreasing curve is created for an average signal received by determination of the average sample values, each of which represents the average value of the values of samples of the same rank,
on the average decreasing curve, the same characteristic values as those recorded in the database are determined,
the point corresponding to the real curve obtained is situated in the N-dimension space of the database,
there is detection if the point of the real curve obtained is within the detection volume, the position and the nature of the object being determined as a function of the neighbouring points in space.